This invention relates to high intensity discharge lamps, more particularly, to methods and apparatus for initiation of the spark discharge in low wattage metal halide lamps which are known to be difficult to start because they require a higher starting voltage than is available from a 115 Volt line and a simple lead-lag ballast.
As described in U.S. Pat. No. 4,353,012, issued Oct. 5, 1982, conventional high intensity discharge metal halide lamps include two main electrodes at opposite ends of a discharge tube and a third starting electrode associated with one of the main electrodes. A starting circuit applies a high voltage between the main electrodes of the lamp and, simultaneously, between the starting electrode and its associated main electrode. A discharge is initiated between the starting electrode and the main electrode by the starting circuit. The discharge then transfers to provide a discharge between the two main electrodes. After a high intensity discharge is formed within the discharge tube, the voltage between the electrodes drops and the starting circuit is no longer operative.
While the starting electrode in metal halide lamps provides generally satisfactory operation, it has certain disadvantages. The complexity and cost of manufacturing the lamp are increased when the starting electrode is used. In addition, the lamp seal in the region of the starting electrode is adversely affected by an electrolysis process when a potential difference exists between the starting electrode and the main electrode. The degradation of the seal can eventually lead to lamp failure. It is known that this problem can be alleviated by connecting a thermal switch, which closes after starting of the lamp, between the main electrode and the starting electrode. However, the thermal switch adds to the overall cost and complexity of the lamp assembly. It is, therefore, desirable to provide a starting arrangement for metal halide lamps wherein the starting electrode can be eliminated.
The spiral line pulse generator, disclosed by R. A. Fitch et al. in U.S. Pat. No. 3,289,015, issued Nov. 29, 1966, is a device capable of storing electrical energy and, upon momentary short circuiting of a pair of terminals, of providing a high amplitude pulse. The spiral line pulse generator can, when properly utilized, provide the dual functions of storage and voltage multiplication. The spiral line pulse generator is a transient field reversal device which provides a roughly triangular pulse. Its peak voltage is a multiple of the initial charging voltage. The use of a spiral line pulse generator to start high pressure sodium lamps is disclosed in U.S. Pat. No. 4,325,004 issued Apr. 13, 1982 and assigned to the assignee of the present application. The output of the spiral line pulse generator is coupled to a conductor, or starting aid, located in close proximity to an outer surface on the central portion of the discharge tube. In the case of metal halide lamps, it has been found undesirable to locate conductors in close proximity to the central portion of the discharge tube, thereby ruling out the use of such a starting aid to assist in initiating discharge.
U.S. Pat. No. 4,353,012 issued Oct. 5, 1982 shows a starting circuit for high intensity discharge metal halide lamps which includes a spiral line pulse generator including two conductors and two insulators, each in the form of an elongated sheet, in an alternating and overlapping arrangement which is rolled together in a spiral configuration having a plurality of turns. The spiral line pulse generator includes an output terminal coupled to one of the electrodes of the lamp and a pair of input terminals. One of the input terminals and the other of the electrodes of the lamp are adapted for coupling to a source of lamp operating power and for delivering lamp operating power, received from the source, through the spiral line pulse generator to the discharge lamp. The starting circuit also includes means for applying a voltage between the conductors of the spiral line pulse generator and for switching the conductors from a first voltage to a second voltage in a time interval much shorter than the transit time of electromagnetic waves through the spiral line pulse generator. After operation of the switch, the spiral line pulse generator provides, at its output terminal, a high voltage, short duration pulse of sufficient energy to initiate discharge in the discharge lamp.
While the starting circuit described in U.S. Pat. No. 4,353,012 is suitable for the purposes intended, its performance could be improved substantially if greater voltage output could be applied to the lamp electrodes, producing a higher breakdown voltage and, thus, more reliable starting.